A current trend in the control of automotive powertrains including automatic shift transmissions is to coordinate the engine and transmission controls for improved durability and shift quality. The engine controls typically include spark timing retardation, fuel reduction, throttle control, or some combination thereof. See, for example, the U.S. Pat. No. 4,724,723 to Lockhart et al., issued Feb. 16, 1988, and Marsh et al. U.S. Pat. No. 4,809,660, issued Mar. 7, 1989, both of which are assigned to General Motors Corporation. In relation to shift quality, the engine torque reduction controls are primarily applied during power-on upshifting to reduce or shorten the output torque disturbance associated with engine inertia.
In the transmission, the process of shifting involves an exchange of on-coming and off-going fluid operated friction elements, generally characterized as comprising three successive phases: a fill phase, a torque phase and an inertia phase. In the fill phase, the on-coming element is prepared for torque transmission; in the torque phase, the torque exchange occurs without a corresponding speed change; and in the inertia phase, the speed change occurs. In shift controls based on an open-loop control philosophy, the fluid pressure supplied to the on-coming element during the torque and inertia phases is progressively increased in accordance with a predetermined pressure schedule, based in part on an estimate of the engine torque applied to the transmission at the initiation of the shift. The scheduled pressure determines the torque capacity of the on-coming element, and therefore, the progression of the shift.
A difficulty arises with the above-described transmission controls during low torque conditions when accessory loading represents a sizable percentage of the torque developed by the engine. Since the transmission input torque cannot be accurately estimated under such conditions, the scheduled pressure may be inappropriate. This is particularly significant if the input torque is underestimated, since the scheduled pressure will be too low, allowing the engine speed to flare.